Technical Field of the Invention
The present invention relates to electrodes for alkaline storage batteries, a method for manufacturing electrodes for alkaline storage batteries, and improvement of alkaline storage batteries.
Background of the Technology
Alkaline storage batteries as represented by nickel-hydrogen batteries and nickel-cadmium batteries, etc., are small, light weight and provides high output densities. In recent years, their field of practical applications is not limited to small devices such as personal computers and mobile phones but is extending to the area of large size power sources such as for electric vehicles and hybrid electric vehicles. In association with the expanding applications of these batteries, increasingly higher capacity and reliability are now being required.
In alkaline storage batteries, a prismatic structure in which a battery is configured by assembling plate-form positive electrodes and negative electrodes with separators interposed and housing in a prismatic container, and a cylindrical structure in which a battery is configured by spirally winding a rectangular positive electrode and a negative electrode with a separator interposed thus configuring an electrode group and housing it in a cylindrical container, are generally adopted.
On the other hand, in recent years, three-dimensional porous metal substrates (hereinafter porous substrates) such as foam metal of nickel or non-woven metal fabric having three-dimensionally connected space with as high a porosity as exceeding 90% have been developed, and a new method for manufacturing electrodes by directly filling paste of active material particles into these substrates has been developed, and these electrodes are now in wide use in the above-mentioned prismatic and cylindrical batteries as the electrodes suited for higher capacity. However, it has become clear that there exist reliability related issues arising from the manufacturing method and structure of the electrodes employing the above-mentioned three-dimensional porous substrate. It is an object of the present invention to improve the electrodes from the standpoint of the structure and the manufacturing method, and to establish reliability of alkaline storage batteries provided with electrodes that employ the above-mentioned three-dimensional porous substrate.
As an active material paste to be filled into a porous substrate, active material paste for the positive electrode of an alkaline storage battery that contains nickel hydroxide as the main constituent added with a material required for electrode reaction such as cobalt metal, nickel metal, and carbon powder, and active material paste for the negative electrode that contains hydrogen absorbing alloy or cadmium hydroxide as the main constituent added with a material such as carbon powder and nickel powder required for electrode reaction, and a binder such as carboxymethyl cellulose and the like are in use.
Electrodes using three-dimensional porous bodies filled with the above-mentioned active materials have been generally used as a positive electrode or a negative electrode of a battery after being pressed into a predetermined thickness after the paste was filled.
It is common that a dense surface layer has been formed on the surface of an electrode pressed as above. Such a dense surface layer hampers penetration of liquid electrolyte into the inside causing dispersion in the quantity of injected liquid electrolyte from battery to battery thus suffering from dispersion in the characteristics.
On the other hand, the above-mentioned three-dimensional porous body is a structural material with an originally low degree of freedom of deformation against bending force. When an electrode fabricated by filling an active material into such a structural material and further pressing the degree of freedom against bending force is further limited. When trying to configure an electrode group by forcefully winding the electrode, disorderly cracks, may occur on the outside of the electrode being wound, or a squarishly wound electrode group with poor roundness is formed, thereby causing failure when inserting into a cylindrical metal container. Furthermore, at the above-mentioned cracks, burrs from the damaged portion may project from the surface or active material particles may flow out from the damaged portion, thus penetrating the separator and causing short-circuits of various sizes, further causing initial or time-varying voltage failures or short-circuit failures.
As a prior art for improving the above issues, a technique has been proposed as disclosed in Japanese Laid-Open Patent Application No. Sho 60-133655, in which V-shaped grooves are formed on both sides of an electrode and winding it with the direction of the grooves in parallel to the axis of winding. Furthermore, in Japanese Laid-Open Patent Application No. Hei 5-41211, disclosure has been made on grooves having a trapezoidal or semi-elliptical cross section instead of grooves having a V-shaped cross section with which a porous metal body is easy to fracture. However, formation of grooves on both sides is disadvantageous in increasing the quantity of active material to be filled.
Accordingly, as a method for manufacturing by uniformly forming grooves by making the distribution of filled active material uniform, one in which active material paste is filled from one side of a three-dimensional porous metal body toward the opposite side has been proposed in Japanese Laid-Open Patent Application No. Hei 9-106814. Also, a technique is disclosed in which a layer filled with a high-density active material is formed on the side of the above-mentioned filled surface by filling the active material in a manner such that it hardly passes to the opposite side, and a layer filled with a low-density active material or a non-filled layer is formed on the opposite side, and grooves are formed on the surface of the low-density filling side. Furthermore, in the same Japanese Laid-Open Patent Application No. Hei 9-106814, a description is made on examples of a method of manufacturing in which grooves or rifts are formed on one side of a three-dimensional porous material body prior to filling paste and a method of manufacturing an electrode in which active material paste is filled from the side opposite to the side on which the grooves or rifts are provided, and further, on a structure in which electrodes are wound in a manner such that the above-mentioned grooves face outward.
On the other hand, in Japanese Laid-Open Patent Application No. Hei 9-27342, an electrode comprising a high-density active material-filled layer and a low-density active material-filled layer similar to the one disclosed in Japanese Laid-Open Patent Application No. Hei 9-106814 is disclosed, and both of a structure made by winding with the low-density active material-filled layer facing inward and a structure made by winding with the layer facing outward are disclosed.
In addition, as an example, a description is made on a method of manufacturing in which grooves or rifts are provided on a three-dimensional porous body prior to the step of filling an active material and active material paste is filled from the side opposite to the side where the grooves or rifts have been provided, and a method of manufacturing in which an active material is filled conversely from the side where the grooves or rifts have been provided.
In either case, electrodes having grooves as described above showed improved flexibility in a configuration in which the electrodes had been wound with the side having grooves facing outward due to freedom of extension of the surface as given by cracks occurring preferentially inside the grooves, and tended to cause fewer voltage failures.
Nevertheless, it has become clear that many voltage failure cases still occur. From the analysis of the causes, it was found that, in the above existing groove forming configuration, burrs of cracks occurring inside the grooves either bulge and project out by the winding force or active material particles from the cracks flow out to the outer periphery of the electrodes passing through the grooves, thus causing new short circuits.
To summarize, it was found that inside of the grooves formed on the surface of a substrate filled with an active material was an empty space, and was unprotected against bulging and projection of burrs due to cracks occurring inside the grooves or against flowing out of active material particles from the cracks, thus creating causes for reduction in reliability.
Also, in an electrode fabricated by forming in advance grooves or rifts on one side of a three-dimensional porous body and filling an active material from the surface provided with the above grooves or rifts, even though the active material is filled in the grooves or rifts, the active material particles filled in the grooves or rifts are fluidized when dampened by a liquid electrolyte and easily flow out from the grooves as the active material existing there is simply an aggregate of active material particles as filled. It was found that an aggregate of the active material fluidized as above did not have any control over bulging and projecting of burrs at cracks and flowing out of the active material, or it rather caused new short circuits.
That is, in order to enhance reliability of alkaline storage batteries that employ electrodes using a three-dimensional porous metal body while attaining a higher capacity, new issues have become clear, especially in cylindrical batteries, such as development of an appropriate electrode structure for controlling bulging and projecting out at cracks that occur inside the grooves and flowing out of active material particles, and a method of fabrication appropriate for manufacturing the electrode structure while improving penetrability of liquid electrolyte into electrodes.
In order to address the above issues, the present invention discloses an electrode for an alkaline storage battery in which grooves are formed on the surface of an active material-filled substrate comprising a porous metal substrate having three-dimensionally connected space and an active material filled in it and, by pressing the active material filled substrate, a coarse in-groove active material layer with a low active material filling density in the grooves and a dense surface layer with a high active material filling density are alternately forming a nearly smooth surface.
The present invention also discloses, as a method of manufacturing for effectively forming the above structure, a method of manufacturing an electrode for an alkaline storage battery that comprises steps of filling an active material in which a filled substrate is formed by filling active material paste into a porous metal substrate having three-dimensionally connected space, forming grooves on one side of the filled substrate, and pressing the groove-formed electrode to obtain a nearly smooth surface.
Furthermore, an alkaline storage battery having an electrode group made by winding an electrode obtained by the above method with the side having the grooves facing outward and with the direction of the grooves and the axis of winding in parallel to each other is disclosed.
The electrode of the present invention has on its surface a coarse in-groove active material layer, and the layer provides a channel for penetration of a liquid electrolyte thereby improving penetrability of the electrode for the liquid electrolyte. Also, although the grooves have apparently disappeared as the surface had been smoothed, an effect of improving flexibility is obtained in a structure in which a plurality of grooves are formed in parallel. Furthermore, as cracks occurring inside the grooves are pressed by the in-groove active material layer formed by pressing, bulging and projection of burrs and flowing out of the active material can be controlled.
Exemplary embodiments of the present invention will be described in the following.
The electrode for an alkaline storage battery of the present invention has a plurality of parallel grooves on one side of an active material-filled substrate comprising a porous metal substrate having three-dimensionally connected space and an active material filled in it. In the grooves, a nearly smooth surface is formed by a coarse in-groove active material layer and a dense surface layer with a high active material filling density formed by pressing.
In the present invention, a high capacity, high flexibility electrode can be obtained by forming an active material layer in the grooves that were empty in the prior arts disclosed in Japanese Laid-Open Patent Applications No. Sho 60-133655 and No. Hei 5-41211. Also, the electrode for an alkaline storage battery of the present invention is characterized by having a coarse and a dense surfaces formed by alternately and regularly repeating in parallel a dense surface layer and a coarse surface formed on the in-groove active material layer, and is superior in absorbing liquid electrolyte. A structure available with the present invention cannot be expected from making grooves or rifts in a porous metal substrate in advance as disclosed in Japanese Laid-Open Patent Applications No. Hei 9-106814 and No. Hei 9-27342.
Furthermore, as the electrode for an alkaline storage battery of the present invention does not have fractured portion on the skeleton: of a porous metal body that is in contact with the in-groove active material layer, occurrence of short circuits due to projection of burrs is controlled when wound as an electrode of an alkaline storage battery while at the same time an electrical conduction channel is maintained, and when used as an electrode of a battery, internal resistance decreases compared to the prior arts and large-current discharge characteristic is improved.
In an example of the manufacturing method to obtain an electrode for an alkaline storage battery of the present invention, the steps of filling an active material in which an active material-filled substrate is formed by filling an active material into a porous metal substrate having three-dimensionally connected space, forming grooves on one side of the active material-filled substrate, and nearly smoothly pressing the electrode formed with the grooves to a predetermined thickness are successively performed.
An electrode for an alkaline storage battery of the present invention with a uniform filled quantity of active material is obtained by filling, in the step of filling an active material, active material paste from one side of a porous metal body in a manner such that it will not penetrate to the opposite side, and forming grooves, in the step of forming grooves, on the side the active material paste is filled.
When configuring a cylindrical alkaline storage battery by using an electrode for alkaline storage battery of the present invention in at least one of the positive electrode and the negative electrode and an electrode group wound with a separator interposed, winding is performed with the side having the in groove active material layer facing outward and the direction of the grooves and the axis of winding in parallel.
During this process, cracks are preferentially formed in the wound electrode starting at the line of intersection of the bottom and the walls of the plurality of parallel grooves. In the present invention, the cracks are not formed on the outer periphery of the electrode group except on the bottom of the grooves, and the depth of the cracks is shallower compared to prior arts thus lowering internal resistance of the battery and improving large-current discharge performance.
Also, the cracks are pressed by the in-groove active material layer, thereby controlling flowing out of the active material and projection of burrs.